1. Field of the Invention
The present invention relates to an image forming technique. More particularly, the present invention relates improvements in a technique of halftoning for converting the gradation of an image into a dot pattern.
The present application is based on Japanese Patent Applications No. Hei. 10-118848 and No. Hei. 11-100881, which are incorporated herein by reference.
2. Description of the Related Art
Concerning a digital image generated or inputted by a computer, a value of density of each pixel is expressed by a word capable of expressing a substantially continuous gradation such as 8-bit word. On the other hand, in an image forming apparatus represented by a digital printer used in a computer system, in general, a pseudo-continuous gradation image, which can be visually assumed to be an image of continuous gradation by human""s eyes, is reproduced when minute points (dots) made of coloring agent of various density and various sizes are put in an image expressing medium. In the above image forming apparatus, it is necessary to provide a technique by which the gradation of an original image can be converted into a dot pattern by which the gradation of the original image can be reproduced as faithfully as possible. This technique is referred to as halftoning.
Conventionally, there are known various techniques of halftoning. Typical examples of techniques of halftoning are an error diffusion method and a dither method. Further, there is provided another technique which will be described as follows. Data of pixel values expressing various dot patterns (screens) corresponding to various gradations are previously stored in a memory, and data of pixel values of an appropriate screen are selectively read out and outputted responding to the original pixel values. This method is referred to as a screen method in this specification hereinafter. This screen method is mainly used for an electrophotographic image forming apparatus such as a laser printer in which printing of high speed is demanded, because the error diffusion method is difficult to be applied to the laser printer.
FIG. 1 is a view showing an example of screen data for the screen method used in a conventional laser printer.
These screen data express a screen for a region of 16xc3x9716 pixels on an image. These screen data are made as a gamma cell table 1 composed of gamma conversion cells 3, the number of which is 16xc3x9716. The individual cells 3 on the gamma cell table 1 correspond to individual pixels in the region of 16xc3x9716 pixels. In each cell 3, 256 laser pulse width values (8 bit words) corresponding to 256 gradation values, which can be taken by the original pixel values, are housed. The printer successively reads out a value of each pixel of the original image from a memory.
In the region of 16xc3x9716 pixels, from one gamma conversion cell 3 on the gamma cell table 1 corresponding to the pixel position, a laser pulse value corresponding to the pixel value is read in. When pulse width modulation is conducted on a drawing laser pulse according to the laser pulse width value which has been read in, a dot, the size of which corresponds to the pulse width value, is drawn at a position of the pixel on a sheet of printing paper. In this way, halftoning is conducted by using a screen of the pixel size of 16xc3x9716.
In order to conduct processing at high speed, screen data are housed in SRAM of high speed. However, according to the conventional technique, since a quantity of screen data is large, it is necessary to provide a large quantity of expensive SRAM, which raises the manufacturing cost. For example, in the case of the above screen, the pixel size of which is 16xc3x9716, SRAM of (8 bxc3x97256 words)xc3x97(16xc3x9716 pixels)=512 kb is consumed. Further, there is a demand for using a larger screen in order to enhance image quality, that is, there is a demand for using a larger screen, the size of which is, for example, 64xc3x9764 pixels. In the case of a screen of 64xc3x9764 pixels, a quantity of data becomes (8 bxc3x97256 words)xc3x97(64xc3x9764 pixels)=about 8 Mb, which is very large.
Therefore, it is an object of the present invention to halftone a large sized screen using a small capacity of memory.
The present invention is based on the following new knowledge. In general, designing is conducted in such a manner that dots grow from small points to large masses according to an increase in the gradation value of an original image, that is, according to an increase in density. Dots grow in the same manner as that of crystals. That is, each dot grows from a center, which is referred to as a growing core, to its periphery. The inventors made investigation into a relation between the growth of dots and the effect of halftoning. As a result of the investigation, the following were found. For example, when there are about 4 growing cores in the case of a screen of 4xc3x974 pixels, or when there are about 8 growing cores in the case of a screen of 8xc3x978 pixels, or when there are about 64 growing cores in the case of a screen of 64xc3x9764 pixels, it is possible to sufficiently exhibit the effect of halftoning which is expected for the screen of the size, that is, it is possible to exhibit the effect of faithfully reproducing the gradation in the pseudo manner. In short, in the case of a screen, the region of which is rectangular (square in a typical case), it is possible to sufficiently exhibit the effect of halftoning when the number of growing cores is approximately a square root of the number of pixels contained in the screen.
The halftoning device of the present invention accomplished according to this knowledge includes a conversion cell table to be used as screen data. This conversion cell table contains a plurality of conversion cells, the number of which is smaller than the number of pixels contained in a pixel matrix of a predetermined size. Conversion information for converting a pixel value of one pixel into a dot signal is housed in each conversion cell. Conversion information has a conversion characteristic for converting a pixel value into a dot signal. The conversion characteristic is different at each conversion cell. When these conversion cells are applied to a plurality of pixels, which form a mass, it is possible to make a dot growing pattern, the center of which is one growing core.
The halftoning device of the present invention comprises:a cell designation section for designating one of the conversion cells on the conversion cell table with respect to each pixel position on the pixel matrix; and a processing section for determining a pixel position on the pixel matrix of each inputted pixel of an inputted image by applying the pixel matrix to the inputted image, the processing section also for converting the pixel value of each inputted pixel into a dot signal by using a conversion cell designated by the cell designation section with respect to the determined pixel position.
According to the method of halftoning of the present invention, a pixel position of each input pixel on the pixel matrix is determined. According to the determined pixel position of each input pixel, one of the plurality of conversion cells contained in the conversion cell table is designated for each input pixel, and the pixel value of each input pixel is converted into a dot signal by using the conversion cell designated for each input pixel.
According to the present invention, the number of conversion cells on the conversion cell table is smaller than the number of the pixels of the pixel matrix which is covered by the screen for halftoning. Therefore, a quantity of screen data is smaller than that of the conventional art. According to the new knowledge described above, in the case of a screen, the pixel size of which is Nxc3x97N, it is possible to provide a sufficiently high effect when the growing cores, the number of which is N, exist. Therefore, it is possible to provide a sufficiently high effect when the conversion cells on the conversion cell table, the number of which is approximately N, exist, wherein N is a number obtained when Nxc3x97N of the screen size is divided by the number N of the growing cores. This means that a quantity of screen data is reduced to about 1/N compared with that of the conventional art.
In a preferred embodiment of the present invention, referring to a lookup table which is referred to as an index table, a conversion cell to be applied to each input pixel is selected. On this index table, there are housed indexes indicating the conversion cells for all pixel positions on the pixel matrix. A quantity of data on the index table is much smaller than that on the conversion cell table.
In another preferred embodiment, conversion cell to be applied to each input pixel is calculated and determined by a pixel position (for example, the line number and the row number) of the input pixel on the pixel matrix. Therefore, the above index table is not required.
Typically, the present invention is carried out by an exclusive hardware, however, of course, it is possible to carry out the present invention by a computer. In this case, the computer program can be installed or loaded into the computer via various media such as a disk type storage, semiconductor memory, correspondence net work and so forth.
Features and advantages of the invention will be evident from the following detailed description of the preferred embodiments described in conjunction with the attached drawings.